1. Field of the Invention
The present invention relates to a knock control apparatus for an internal combustion engine, and more specifically, to a method of setting a knock determination level.
2. Description of the Related Art
In general, a knock control system judges the occurrence of knock when an electric signal from a knock sensor for detecting a vibration of an engine (hereinafter, referred to as a knock sensor signal) exceeds a predetermined level (hereinafter, referred to as a knock determination level) to retard ignition timing. On the other hand, when the knock is not detected for a predetermined period of time, the knock control system advances the ignition timing to constantly control the ignition timing to be in the vicinity of a knock limit. In this manner, the maximum fuel efficiency and output characteristics of the engine are obtained.
In the knock control system as described above, the knock determination level has a great importance. Specifically, when the knock determination level is too high, the knock is not detected even though the knock actually occurs. Therefore, the ignition timing is advanced to cause frequent knocking, which leads to damage to the engine. On the other hand, when the knock determination level is too low, the knock is erroneously detected even though the knock does not actually occur. As a result, the ignition timing is retarded, failing to obtain a sufficiently high output of the engine.
As a method of setting the knock determination level, for example, there is known a method involving multiplying an average value of peak hold values of the knock sensor signals by a gain closely pre-matched for each revolution of the engine and then adding an offset voltage to the result of multiplication to obtain an appropriate knock determination level.
Besides the above-mentioned method, a technology of correcting the knock determination level based on a lognormal distribution obtained by logarithmic transformation of a maximum value of the output signal from the knock sensor to precisely judge the occurrence/non-occurrence of the knock has also been proposed (For example, Japanese Patent Application Laid-open No. 2007-9725).
In the conventionally known methods of setting the knock determination level, however, a considerably large number of steps are required for the gain or offset matching. Moreover, each of the internal combustion engines has a manufacturing error even when the internal combustion engines are of the same type. Therefore, there is a problem in that the knock determination level is set to an inappropriate value to prevent precise knock detection from being performed even though the gain or the offset is closely matched.
Further, in the technology disclosed in Japanese Patent Application Laid-open No. 2007-9725, the knock sensor signal is subjected to statistical processing after being subjected to the logarithmic transformation. Therefore, when the distribution of the knock sensor signal does not become the lognormal distribution, there is a problem that the knock determination level is set to an inappropriate value to prevent precise knock detection from being performed.